The present invention is directed to a blank for producing a formed dental part, which blank comprises a blank member of a tooth restoration material from which the formed part can be produced with a tool, a retaining projection for holding the blank member in the processing machine and a reference face having close tolerances for engagement by the tool for the purpose of calibration of the tool.
U.S. Pat. No. 4,615,678, whose disclosure is incorporated herein by reference thereto and which claims priority from the same Swiss Application as European Patent 0 160 797, discloses a blank, which can be machined or formed into a dental part. The blank is composed of the blank member of the tooth restoration material from which the actual dental part, such as an in lay, on lay, bridge or the like, can be produced and, in addition, has a retainer projection connected to the blank member. The purpose of the retainer projection is always to clamp the blank in an unambiguous position in the processing means or machine on the basis of a stop face and to be able to simultaneously exactly center the blank. To this end, the retainer projection contains at least one stop face for defining the angular position and, at its end face toward the blank member, a reference face that is arranged concentrically relative to the cylindrical axis and is exposed for the grinding tool and is cylindrical at least in sections. Within a narrow tolerance, the reference face has a predetermined, radial spacing from the axis of the retainer projection and serves the purpose of being able to define the position and critical dimensions of the processing tool, for example, the diameter of a grinding disk. This calibration, i.e., the definition of the reference point of the tool, occurs in that the tool is brought toward or, respectively, touches the calibration location with a small tolerance reference face before the beginning of the process.
In the known blank, the retainer projection must be very precisely fabricated in order to be able to adhere to the required position of xc2x10.01 mm. In terms of fabrication, thus, the retainer projection is of a high-quality part, so that simple manufacturing methods, such as, for example, injection molding, impact molding, die-casting, metal sintering or the like, therefore, do not come into consideration for the manufacture of the retainer.
The present invention is directed to an object of providing a blank wherein the retaining projection can be more economically produced, but the procession of the required reference or, respectively, touching faces can be enhanced given minimization of the manufacturing cost.
To obtain this object, the present invention is directed to a blank for producing a formed part that can be used for replacement or restoration of one or more teeth by removing material with at least one tool chucked in a processing machine, with the blank consisting of a blank member of the tooth restoration material from which the formed part can be worked with the tool, a retainer projection for holding the blank member in the processing machine being positioned at one end of the blank member, and a reference face being arranged at an end of the blank member opposite the retainer projection and containing at least two face elements or surfaces having a defined spacing from one another with a close tolerance.
In that the functions of retaining, on the one hand, and the touching of the reference faces, on the other hand, are separated from one another in the inventively manufactured blank, the retainer projection itself can be very cost-beneficially produced. That part on the end of the blank member lying opposite the retainer projection and containing the reference faces or surfaces, however, can also be comparatively cost-beneficially manufactured. The reference part can, thus, be advantageously cut to length from a rod material. Alternatively, a precision-fit pin or a roller bearing can be employed. The part can be put in place on the blank member without an exact positioning relative to the cylindrical axis of the retainer projection being necessary. As the simplest solution, it is proposed to provide a circular-cylindrical set part. However, it is conceivable and also lies within the framework of the invention to employ a cuboid or similarly-fashioned part. More than two surfaces can thus be potentially required for the calibration, given a lower symmetry of the set part.
The invention is based on the perception that the rotational axis of the processing machine is the only important reference quantity for the tool position and size. Differing from previous practice, the retainer projection need not be utilized for defining the size and position definition of the tool, which usually occurs by approaching or touching at least two reference faces having close tolerances, and this retainer projection need not be manufactured in a relatively complicated manner. On the contrary, a set part independent of the retainer projection suffices, and only this part needs to be constructed with close tolerances. This set part can then be secured to the blank member of the material without adhering to greater dimensional accuracy. Even though the set part need only comprise two reference faces lying opposite one another in the simplest case, with these faces or surfaces having a defined spacing from one another, it is expedient and advantageous to provide a cylindrical set part having a circular cross section. The position and critical dimensions of the processing tool, for example the exact current diameter of the grinding disk or, respectively, the spacing of the grinding disk or an end-milling cutter from the rotational axis of the processing machine, can thus be easily mathematically determined given the known spacing that the two reference surfaces have from one another.
Other advantages and features of the invention will be readily apparent from the following description of the preferred embodiments, the drawings and claims.